Such nice precise work! I had to smile when you said this was for your grandma. Here we are making quilts for our grandmothers. Didn't it used to be the other way round? I'm so glad the art of quiltmaking is alive and well again.
And I agree, sashing and cornerstones would really set those stars off. You might even consider using your background fabric for sashing, with either the yellow or blue (or alternate both) for cornerstones. It would make the stars "float". Just a thought.